Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive collapse or narrowing of the upper airway passages during sleep that impairs ventilation and disrupts sleep. Factors that contribute to upper airway collapse include reduced upper-airway dilator muscle activity during sleep, specific upper-airway anatomical features, decreased end-expiratory lung volume, ventilatory control instability, and sleep-state instability. A collapse or narrowing of the airway passages during sleep may result in total or near total cessation of breathing or a partial reduction of ventilation.
Total or near total cessation of breathing that lasts at least ten seconds is referred to as “apnea”, and typically results in neurological arousal of the person from sleep that initiates activity to reopen the upper airway passages and reestablish breathing. A partial obstruction of the airway passages can lead to a partial reduction of normal airflow during breathing by at least 50% for at least ten seconds, and is accompanied by oxygen desaturation of blood by at least 4%, and/or arousal from sleep is referred to as “hypopnea”. In a vast majority of cases OSA is accompanied by snoring, which is caused by vibration of soft tissue in the upper airway passages.
OSA is associated with an increased risk of cardiovascular disease, stroke, high blood pressure, arrhythmia and diabetes. Sleep fragmentation resulting from obstructive events can also increase a person's risk of being involved in an accident, such as a driving accident as a result of excessive daytime sleepiness and fatigue. Once diagnosed, a number of different therapies are available for treating OSA. The therapies include behavioral modification training, use of masks for introducing a flow of pressurized air into the throat to prevent collapse of tissue in the upper airway passages, and surgery to modify anatomical features of the airway passages that are responsible for OSA.
Diagnosis of OSA and determination of OSA severity are typically made with reference to an index referred to as an apnea-hypopnea index (AHI). The index is simply a count of the number of apnea and hypopnea events that a person exhibits per hour of sleep. An AHI index that is less than about 10 e/hr (events per hour) is usually considered clinically insignificant. An AHI index between about 10 e/hr and about 30 e/hr is considered to indicate a moderate case of OSA, and an AHI index greater than about 30, is considered to indicate a severe case of OSA.
Whereas the AHI index appears simple and straightforward, determining an AHI value for a patient generally involves performing a sleep study, referred to as polysomnography, (PSG) study. PSG is a relatively complicated and expensive procedure carried out in a sleep laboratory during the patient's overnight stay in the laboratory. PSG typically involves attaching a variety of sensors to the patient's body to track changes that occur in a battery of physiological activities and functions such as brain activity, eye motion, skeletal muscle activation, and heart rhythm during sleep. The waiting period for PSG has been reported to be a few weeks to more than a year in the United States.